Polymers, in particular, lattices or polymer particles obtained therefrom are widely used as coatings, adhesives, ink and painting materials, for precision mold constructions and the manufacture of micro-sized materials.
For the latter, the unique properties of micro- and nanoscaled polymer particles with specific properties such as defined molecular weight distributions and polydispersities have meanwhile gained further significant attention not only in the electronics industry, for example, in the manufacture of TFT and LCD displays, digital toners and e-paper, but also in the medical sector such as for drug delivery systems, diagnostic sensors, contrast agents and many other fields of industry.
Polymer nanoparticles are frequently synthesized by physical methods like evaporation of polymer solution droplets or, in particular, for commercially important polymers such as polystyrene and poly(meth)acrylates, by direct synthesis of nanoparticles using special polymerisation processes. The most common processes are heterophase polymerisations, in particular, thermally or photo-initiated emulsion polymerizations.
Over the last decades, efforts were made to develop heterophase polymerization processes and/or post polymerisation modifications in order to achieve better control over molecular weight distribution, particle size of primary polymer nanoparticles, the crosslinking behavior of polymers or polymer nanoparticles or the introduction of polymer end groups.
Emulsion polymerisations induced by X-ray radiation are described in S. Wang, X. Wang, Z. Zhang, Eur. Polym. J., 2007, 43, 178.
Emulsion polymerisations induced by UV/Vis radiation are described in P. Kuo, N. Turro, Macromolecules 1987, 20, 1216-1221, wherein the formation of polystyrene nanoparticles having a weight average molecular weight of 500 kg/mol or less is disclosed.
In T. Ott, Dissertation ETH Zürich No. 18055, 2008, Chapter 6, batch emulsion polymerisations induced by photofragmentation of bisacylphosphines are investigated in detail. However, high monomer conversion typically requires irradiation times of more than 2 hours.
A. Chemtob et al. describes a batch process (in a cuvette) for the preparation of lattices comprising copolymers by irradiating a miniemulsion of nanodroplets comprising acrylic acid, butylacrylate and methylmethacrylate encapsulating high amounts (4 wt.-%) of a hydrophobic photoinitiator of the BAPO type (BAPO=bisacylphosphine oxide).
WO 2005/042591 A describes a process for preparing polymers with defined end group functionalities which comprises polymerizing monomers in a heterophase medium and in the presence of a water-soluble photoinitiator system bearing said desired end group. The end group is deliberated as starter radical for chain propagation upon photofragmentation of the photoinitiator.
A major disadvantage is the use of methylene blue or other colored and/or toxic compounds and the low variability of end groups.
EP 1 300 427 A describes a process for preparing hot melt adhesives which comprises polymerisation of acrylates in the presence of common photoinitiators and control agents such as thioesters, trithiocarbonates or dithioesters in order to provide enhanced crosslinking ability upon UV-irradiation.
A major disadvantage is the use of high amounts of different compounds necessary to achieve the desired properties.